Apparatus for making geophysical explorations



Feb. 29, 1944. H EVJEN 2,342,626

APPARATUS FOR MAKING GEOPHYSICAL EXPLORATIONS Filed Jan. 8, 1942 2Sheets-Sheet 1 1 1; 41 I 5? v 38 E;

INVENT OR. HAA KO/V M. El/dE/V A TTORNE X H. M. EVJEN Feb. 29, 1944.

APiARA'IUS FOR MAKING GEmHYsIcA L EiPLoRATIoNs 2 Sheets-Sheet 2 FiledJan. 8, 1942 Patented Feb. 29, 1944 APPARATUS FOR MAKING GEOPHYSICALEXPLORATIONS Haakon M. Evjen, Houston, Tex., assignor to NordelCorporation, Houston, Tex., a corporation of Delaware ApplicationJanuary 8, 1942, Serial No. 425,991

4 Claims This invention rela s to a method of electrical prospecting andto an apparatus for carrying out the same and is an improvement on thesystem disclosed in a patent to Knute Evjen, No. 2,294,395 dated Sept.1, 1942, which embodies an earth current circuit including a currentsource and a controlled reversing means for passing a current reversedat a predetermined controlled low frequency through the earth betweenspaced earth electrodes, and a potential circuit including potentialpick-up electrodes located at spaced points adjacent the earths surfaceand lying within the field of influence of the earth current. Thepotential circuit is provided with reversing means synchronized with thefirst reversing means for converting the picked-up potential into auni-directional potential suitable for actuating adirect currentmeasuring instrument such as a galvanometer. The earth current circuitand the potential circuit are coupled by a coupling circuit whichincludes means for producing a potential drop proportional to the totalcurrent flowing in the earth current circuit and for balancing acontrollable portion of this potential drop against the picked-uppotential so as to obtain, by a single reading, a measurement whichindicates directly the earths apparent resistivity.

An object of this invention is to provide a novel and improved system ofthe type above indicated.

Another object is to provide a novel and improved coupling circuit in asystem of the above type.

Various other objects and advantages will be apparent as the nature ofthe invention is more fully disclosed.

Although the novel features which are believed to be characteristic ofthis invention are more particularly pointed out in the claims appendedhereto, the invention itself may be better understood by referring tothe following description, taken in connection with the accompanyingdrawings, in which certain specific embodiments thereof have been setforth for purposes of illustration.

In the drawings,

Fig. 1 is a schematic representation of an electrical prospecting systemembodying the present invention; and

Figs. 2 and 3 are similar schematic representations illustratingdifferent embodiments of the invention.

The present invention provides a novel and improved meansgfor obtaininga measuring potential which bears a definite relationship to the currentflowing in the earth circuit and may be compared with the potentialimpressed between the measuring or potential electrodes by theearthcurrent flow. This is accomplished by utilizing a motor-generatorincluding D. C. generator which is driven at constant speed, as by aconstant speed motor, and the field winding of which is connected inseries with the earth current circuit. The generator is of a typeproducing a current proportional to field excitation, such, for example,as a generator without iron in its magnetic circuits or one having anair gap in the magnetic circuit suited to produce the above effect. TheD. C. generator supplies direct current to a potentiometer to develop apotential drop therein which is directly proportional to the fieldexcitation and hence to ,the earth current. The measuring potential istaken from the potentiometer and may be adjusted to exactlycounter-balance the potential being measured. A galvanometer or the likemay be used for determining when an exact balance is reached.

The only reading which is required in this system is the setting of thepotentiometer by which the measuring potential is obtained. Thispotentiometer may be calibrated in suitable units which correspond tothe earths electrical characteristics and may be interpreted to indicatethe variations in the earths apparent resistivity without furthercalculations.

One embodiment of this invention is shown more in detail in Fig. 1.Referring to this figure, the earth current circuit comprises a sourceID of direct current, such as a generator or battery. The source I0 isconnected in series circuit with a line H, the field winding I2 of a D.C. generator 25, an ammeter I3 and a line H. Lines II and I4 areconnected to segments l5 and I6 respectively of a commutator ll which isdriven by suitable means to be described. Alternating current is takenfrom the commutator I! by means of lines l8 and I9 respectively whichare connected to suitable ground electrodes 20 and 2| herein referred tofor convenience as current electrodes. The current electrodes 20 and 2|are spaced a convenient distance apart, which may be of the order ofseveral thousand feet, and when the commutator I! is driven at asuitable speed, an alternating current having a frequency correspondingto the speed of rotation of the commutator, passes through the earthcircuit. This current may be measured by the ammeter l3 and may beadjusted within desired limits by suitable regulation of the source It).

The armature 26 of the D. C. generator 25 is connected by lines 21 andresistances 38 and lines 38a. across a potentiometer 39 having variablecontacts 40 and 4|. The generator 25 is driven at a constant speed by aconstant speed motor 28 of any suitable type. The motor 28 may beenergized from the generator l0 if desired. The contact 40 is connectedby a line 42 to a galvanometer 43 and thence through a line 44 to acommutator 46. The contact 4| is connected by a line 41 to saidcommutator 46.

Segment 45 of the commutator 46 is connected by a line 50 to an earthpotential electrode box and a rheostat for adjusting the motor speed. Acontrol handle 63 is provided for controlling the motor speed asdesired. The speed of rotation of the commutator may be indicated by aspeed indicator 64.

The galvanometer 43 is preferably of the highly damped type so that itis influenced only by the direct current component and not by any rippleor other alternating current component in the circuit under measurement.

Operation In the operation of this system, the current electrodes 20 and2| are located at selected points in the earth and the potentialelectrodes 5| and 51 are located at points within the field of influenceof the earth currents produced between the current electrodes. Thesource it is adjusted to pass a predetermined amount of current throughthe earth circuit, as indicated by the ammeter 13. The frequency ofalternation of the current is regulated by making suitable adjustment ofthe control handle 63. If the measurements are to be taken with directcurrent, the commutator H remains stationary. For low frequencyalternating current, the commutator is slowly rotated at the selectedspeeds as indicated by the speed indicator 5 3. The potential generatedby the generator 25 is proportional to the field excitation which, inturn, is proportional to the current flowing in the earth currentcircuit. A potential drop is thus developed across the potentiometer aswhich is at all times proportional to the current flowing in the earthcurrent circuit.

With the circuit operating in this manner, suitable adjustments are madeof the contacts 30 and ll to produce a zero reading on the galvanometer43 which indicates that the average voltage drop across the intermediateportion of the potentiometer 39 between the contacts 30 and dl exactlybalances the potential which is picked up by the earth potentialelectrodes 5| and 57 after the latter has been rectified by commutator36.

The potential picked up by the potential electrodes corresponds infrequency to the frequency of the earth current. The alternatingpotential is rectified by the commutator 36 which operates insynchronism with earth current commutator H to supply a direct voltageacross the galvanometer 43 and to the potentiometer 39.

In the above described system a definite component of the picked-uppotential is utilized. The measuring potential is in phase with theearth current whereas the picked-up potential has, in general, a phaseshift. By means of commutator 46 a component of this potential, having adefinite phase relation to the current, is selected. The phase relationof this component to the current is determined by the phase relationbetween commutators I7 and 46. In

- the accompanying figures, a phase angle of 0 is shown; but it is tobe? understood thatany, de-i I sired Phase angle may'be usedand'thatthls angle may be changed as desired" This subi'ect I matter isbeing claimed injmyico pending application Ser. No. 477,015, med r etfzs, 1943.

The effect of any residual earth current, which wouldtend to produce adirect potential across the electrodes 5| and 51, is eliminated bysuitable adjustment of the potentiometer 54. This adjustment may bemade, for example, while no current is flowing through the earth circuitso that the only effect on the galvanometer 43 represents the effect ofthe residual earth current. Since the earth current impresses a directcurrent potential across the probe (or potential) electrodes, anyunbalance between this potential and the potential taken frompotentiometer 54, will appear as an alternating potential aftercommutation by commutator 46. This will make the galvanometer kick fromone side to the other in synchronism with the commutation. Any drift inthe natural earth potential may thus be immediately perceived, and canbe compensated by readjusting potentiometer 54 until the kicking of thegalvanometer is eliminated.

A filter may be included in the measuring circuit if desired in order toeliminate ripples of fluctuations from various causes such, for example,as those produced by the action of the commutators.

The potentiometer39 may be suitably graduated in convenient units andwhen the balance is obtained, the reading corresponding to the point ofadjustment is noted. The frequency is also noted from the indicator 64.The potential developed across the potentiometer 39, being dependentupon the total value of the earth current at the instant that themeasurement is being taken, automatically compensates for any variationsin electrode resistance or in contact resistance and eliminates anynecessity for making simultaneous measurements of the earth current andthe induced potential.

This measurement is repeated with different frequencies until a completepotential spectrum of the area between electrodes 5! and 57 is obtained.These electrodes may then be moved to another location-and themeasurements repeated to obtain a potential spectrum at variousfrequencies in the manner above pointed out. The various spectra thusobtained may be interpreted to show the characteristics of the earthsstrata in the area under investigation. It is to be understood, ofcourse, that the potential electrodes may be variously positioned withrespect to the current electrodes 20 and 2| and that the positionsthereof may be changed in accordance with the depth which is underinvestigation at any particular instant.

It will be noted that in the above described system only a singlereading need be taken and the necessity for mathematical calculations isavoided. The system is accordingly well adapted to field use and may beoperated by unskilled persons. The data thus compiled may be interpretedby geological physicists in accordance with well known principles.

The size of the electrodes may vary over wide limits. In practice, metalstakes about one or two inches in diameter and 3 ft. or 4 ft. in lengthhave been found satisfactory. The distance between the currentelectrodes may vary widely, for example, from 200 ft. to 5000 ft,depending upon the depth to be investigated. The potential of the sourcell may likewise be adjusted as desired. Voltages of 100 volts to 200voltsand earth cur,- rents of one to two amperes have been foundsatisfactory.

In the embodiment shown in Fig. 2 parts corresponding to Fig. 1 aregiven the same reference numbers. The coupling circuit indicated by therectangle N is identical with the coupling circuit included in therectangle 24 of Fig. 1. It is to be noted that the measuring voltage istaken from the direct current side of the current commutator II as inFig. 1, but is applied in Fig. 2 to the alternating current side of thecommutator 46.

In Fig. 2 the segment 48 of the-commutator ll is connected by a brush IIand lead 84 to the contact 53 of the potentiometer as. The galvanometer43 is connected by leads 85 and l and brushes 81 and 8| to thecommutator 40. The lines 380 are connected by brushes Ill and II tosegments I2 and I! respectively of a commutator I4 which is mounted onthe shaft and is driven in synchronism with the commutators i1 and 48.The commutator i4. is connected by lines 15 and 18 across apotentiometer ll having variable contacts I! and 8|. The contact I! isconnected by a line" and brush 82a to the segment 45 of the commutator48. The contact 80 is connected by a line 8| to the potential electrodeii. The remaining elements are identical with those shown and describedin Fig. 1.

In this embodiment the measuring potential derived from the generator 28is. reversed by means of commutator 14 in synchronism with the reversalsof the earth current and is applied to the potential circuit on thealternating current side of the commutator ll. The potential derivedfrom the potential electrodes SI and I1 is balanced against thepotential taken from the potentiometer I0, is rectified by thecommutator ll and is applied as a directcurrent to the galvanometer 43.The operation is otherwise identical with the system shown in l ig. 1.

In the embodiment shown in Fig. 3 the various parts are similar to Fig.1 and, given corresponding reference characters. In this embodiment,however, the coupling circuit indicated by the rectangle 24 is connectedbetween the alternating current sides of thecommutators l1 and 40. In

this embodiment the ammeter I3 is connected by the lead ll directly tosegment ll of the commutator H. The iield winding I2 is connected in thelead is. The potentiometer II is connected as shown in Fig. 2 to theleads II and 82 between the commutator it and the potential electrodeIi. rue other elements are connected as previously described and operatein a similar manner.

It is to be understood that the commutators ll. 40 and 14 are shown asrotary commutators merely for purposes of illustration and that anysuitable synchronised reversing means may be used for reversing thecurrent in tlie'respective circuits so as to obtain the balancingandmeasuring eflect above descri Furthermore, any suitable means may beused for controlling the rate of operation of such commutators forthereby varying the frequency of the measuring current so as to obtainthe desired frequency spectrum of the various localities undermeasurement.

Although certain speciiic embodiments of the invention have been shownfor Purposes of illustration, it is to be understood that the inventiona not to be limited thereto but that various changes and modificationsmay be made therein as will readily appear to a person skilled in theart.

5 The invention is only to be limited in accordance with the scope ofthe following claims. What is claimed is:

1. A system for electrical prospecting, comprising an earth currentcircuit including a source of direct current, a pair of spaced earthelectrodes, and reversing means connected between said source and saidelectrodes for reversing the direct current at a predeterminedcontrolled frequency whereby an earth current having a predeterminedfrequency is passed through the earth between said electrodes, apotential circuit including potential pick-up electrodes located atspaced points lying within the field of influence of said earth current,a direct current indicating device 2 connected to indicate the potentialof said potential circuit, and reversing means synchronized with saidreversing means and connected between said potential pick-up electrodesand said indicating device to convert the picked-up potential diil'renceinto a unidirectional potential suited to actuate said indicatingdevice, and coupling means connected between said earth current circuitand said potential circuit to impress on said potential circuit abalancing potential which is directly proportional to the currentflowing in said earth current circuit at the instant of measurement,said coupling means comprising a motor-generator including a directcurrent generator having a field winding connected in series with saidearth current circuit and having an armature driven at substantiallyconstant speed by said motor, said generator being of the type supplyinga current which is proportional to field excitation, and a potentiometerconnected to be fed by said generator to develop a potential dropproportional to the current flowing in said earth current circuit, saidpotentiometer being connected in said potential circuit to apply theretoa potential drop opposing the potential picked up from said potentialpick-up electrodes.

2. The system set forth in claim 1 in which said neld'winding isconnected on the direct current side of the reversing means of saidearth current circuit and said potentiometer is connected on the directcurrent side of the reversing means of said potential circuit. 1 V

3. The system set forth in claim 1 in which said ileld winding isconnected on the alternating current side of the reversing means of saidearth current circuit and said potentiometer is connected on thealternating current side of the reversing means of said potentialcircuit.

4,- The system set forth in claim 1 in which said field winding isconnected on the direct current side of the reversing means of saidearth current circuit, the potentiometer is connected on the alternatingcurrent side of the reversing means of manor: a. swan.

